The present invention relates to capacitive energy storage devices. The storage devices of the present invention may be used at cryogenic temperatures.
Storing energy in banks of capacitors at room temperature is commonly used in applications where size or weight has not been a major concern. Such capacitor technology is quite advanced. However, where the size and weight of the capacitive energy storage devices are of significance and are desired to be minimized, and where rapid discharge is desired, there are potential benefits to be obtained by improvements in capacitive energy storage device structure.
In capacitive energy storage devices, the Helmholtz free energy density of the dielectric is an important quantity. The larger the Helmholtz free energy density of the dielectric, the greater the energy per unit volume which can be stored. The Helmholtz free energy density is defined by the following equation:             Δ      ⁢              xe2x80x83            ⁢      F        =                  1                  8          ⁢                      xe2x80x83                    ⁢          Π                    ⁢                        ∫          0                      E            c            2                          ⁢                  ϵ          ⁢                      xe2x80x83                    ⁢                      ⅆ                                          E                2                            ⁡                              (                cgs                )                                                          ,
where F is the Helmholtz free energy density, xcex5 is the dielectric constant of the material, E is the electric field strength, and Ec is the upper limit of electric field strength.
Some studies of capacitive energy storage at cryogenic temperatures have been published. One study dealt with the impregnation of dielectric films with liquid nitrogen or polar liquids. K. N. Mathes and S. H. Minnich, xe2x80x9cCryogenic Capacitor Investigation,xe2x80x9d Final Report, S-67-1095, May 1965. Three types of materials were investigated at 77 K, and it was concluded that energy densities of approximately 0.6 J/cm3 were possible. Energy density may be defined as the energy per unit volume of a medium.
The use of strontium titanate glass ceramic materials as capacitive energy storage devices at cryogenic temperatures was reported by Lawless, Proc. XIII Int""l. Congress of Refrigeration, Washington, D.C., 1971, Vol. 1, p. 599. Based on measurements of electric field strength and dielectric breakdown at 77 K, it was predicted that energy densities of approximately 5.0 J/cm3 were possible.
However, there is a need in the art for materials which can be used as capacitive energy storage devices and which have even greater energy densities. The size and weight of capacitive energy storage devices could be reduced, providing portability to devices which have been heretofore too large and bulky to be mobile. For example, high powered lasers require massive capacitor banks which are too large and heavy to be moved easily. Capacitive devices having large energy densities could reduce the necessary bulk of the capacitors presently utilized in such applications.
U.S. Pat. No. 4,599,677, CAPACITIVE ENERGY STORAGE DEVICE FOR USE AT CRYOGENIC TEMPERATURES, issued Jul. 8,1986, the disclosure of which is incorporated herein by reference, teaches a capacitive energy storage device utilizing ceramic materials found to possess large dielectric constants at temperatures in the range of about 50 K to 90 K.
However, even in view of the significant advances introduced by the capacitive energy storage devices described in U.S. Pat. No. 4,599,677, there exists a continuing demand for energy storage devices having improved operating characteristics.
This demand is met by the present invention wherein a capacitive energy storage device is provided comprising specialized dielectric materials.
In accordance with one embodiment of the present invention, a multilayer structure capacitive energy storage device is provided comprising a plurality of electrode layers and a plurality of dielectric layers. Respective ones of the dielectric layers are interposed between a selected pair of the plurality of electrode layers. The dielectric layers comprise a combination of materials as follows
PbMgxNbyOz+SrTiO3.
Accordingly, it is an object of the present invention to provide an improved capacitive energy storage device having improved operational characteristics through proper selection of a material for forming the dielectric layers of the device. Other objects of the present invention will be apparent in light of the description of the invention embodied herein.